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Norwegians can now heat their homes and offices by flushing the toilet.

The sewage heat pump plant, which began operating last week, uses fridge technology to tap heat from raw sewage and direct it back to the country's capital Oslo.

Machines at the end of a 300 metre long tunnel in a hillside in central Oslo suck heat from the sewer and transfer it to a network of hot water pipes feeding thousands of radiators and taps in the city.

Similar systems have been used in other parts of the world but the Norwegians say theirs is the biggest.

"We believe this is the biggest heating system in the world using raw sewage," says Lars-Anders Loervik, managing director of Oslo energy company Viken Fjernvarme, which runs the plant.

The pump uses a system of compressors and condensers and generates 18 megawatts, enough heat to warm 9000 flats or save 6000 tonnes of oil a year, he says.

Exploiting sewage

Untreated sewage from toilets, bathtubs, sinks and rainwater from the streets flows into the system at 9.6°C and comes out at 5.7°C after heat is extracted with a refrigerant.

The energy goes to warming to about 90°C the water in a 400 kilometre pipe system fed to offices and homes, from a temperature of 52°C when it reaches the sewerage plant.

Industrial waste burning plants also heat the water.

About a third of the heat energy comes from electricity driving the system and the other two-thirds is the heat from the sewer.

Dr Monica Axell, head of the International Energy Agency's heat pump centre, says the concept could be a feasible solution for many cities as long as they have the necessary infrastructure.

The only problem with the system is that flow in the sewers tends to be irregular.

Monday mornings between 4am and 6am are especially dry in Oslo because residents go to bed early on Sunday.

But on weekends the flow is good, says project manager for the Oslo plant Oyvind Nilsen.

"When people have been out to parties there's a lot of beer going into the sewer," he says.

Among other sewage energy projects worldwide, US scientists are looking to generate electricity from sewage-eating bacteria.

"The microbial fuel cell work is going well, but we still are not out of the lab on this technology," says Professor Bruce Logan of Pennsylvania State University.